The present invention relates to pyroelectric and piezoelectric materials and more particularly to such materials derived from synthetic organic polymers.
Traditional ceramic or inorganic piezoelectric materials include quartz, Rochell's salt, barium titanate, and the like. Such ceramic materials are a great improvement over the original carnuba wax/rosin electret formation of Eguchi. Thus, initial commercial piezoelectric materials have been limited to piezoelectric ceramics. Recent advances in synthetic organic polymers, though, have resulted in the development of materials which have some practicality in commercial use. Polymers or plastics can be formulated in very thin films having larger surface area and relatively easy to work with on an industrial scale. Accordingly, considerable research has been devoted to investigating various polymers relative to their piezoelectric and pyroelectric behavior especially of electro-acoustical transducers. Typical studies concentrate on the ability of such organic polymers to form thermoelectrets (i.e. an electret roughly is an electrical equivalent of a permanent magnet). The semi-permanent polarization of polymers to create electrets typically is conducted by cooling the material under an applied voltage or electric field. Apparently, the orientation of the substituents on the polymer backbone during cooling of the polymer from a temperature greater than the glass transition temperature (T.sub.g), and especially of substituents with large dipole moments, creates a crystalline-effect or orientation such that a net piezoelectricity is observed. Polymers intensely investigated have included polyvinyl chloride (PVC), polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), nylon, polymethyl methacrylate and other acrylics, and the like. Theoretical explanations correlating the piezoelectric behavior of such polymers have considered alternating dipoles, dielectric relaxation, crystallinity, polymer orientation, dipole moment, bond rotation, side chain orientation, mechanical distortion of oriented dipoles, and similar considerations. None of these correlations, however, satisfactorily explains the results researchers have reached to date, much less provides a unifying theory between ceramic and organic piezoelectricity. Further information on piezoelectric polymer development can be found in the reports by Murayama et al., "The Strong Piezoelectricity in Polyvinylidene Fluoride (PVDF)," Ultrasonics, pp. 15-23 (January, 1976) and by Conlon, "Piezoelectric Nitrile Copolymer," Sandia Laboratories Report SAND 760222, Sandia Laboratories, Albuquerque, N.M. (May, 1975).
The present invention provides new piezoelectric polymeric materials which display piezoelectricity comparable to prior reported polymeric materials and which displayed unusually good pyroelectricity also. Additionally, a remarkable unifying theory between ceramic and organic piezoelectric materials has been developed.